1. Field of the Invention
The present invention relates to an integrated circuit, more particularly, to an integrated circuit having Schmitt input circuits.
2. Description of the Related Art
It is known that, when an input signal which varies slowly (e.g. psec order) is input into an integrated circuit with a an signal threshold type input circuit, a phenomenon (hazard) or an abnormality occurs. Thus, most integrated circuits-are provided with Schmitt trigger input circuits in order to avoid this hazard.
When an integrated circuit having Schmitt input circuits is fabricated, it is necessary to test whether the hysteresis is acceptable, namely, whether the threshold voltages at a H (high) side/a L (low) side of each Schmitt input circuit are acceptable values or not. Conventionally, the test, which is a so-called functional test, is performed by operating internal logic circuits. The the H side threshold voltage (V.sub.TH) of a Schmitt input circuit is measured by repeating the functional test while the level of the signal inputted to the Schmitt input circuit is increased in steps by a predetermined step amount (e.g. 0.1 V) from the GND level. Then, it is determined whether the signal inputted to the Schmitt circuit is recognized as "H" or as "L". The input signal level at which the recognized result changes from "L" to "H" is defined as the V.sub.TH. The L side threshold voltage (V .sub.TL) is measured in a like functional test is repeated while the level of the signal inputted to the Schmitt input circuit is lowered.
As above described, in the conventional integrated circuit, the repeated functional test is required to test the Schmitt input circuit. Therefore, there is a problem that it takes a long time to test the Schmitt input circuit. Particularly, in a large standardized LSI such as a micro processor or a micro controller, the "functional test" is complex and takes a long time. Therefore, it takes a very long time to complete testing of all Schmitt input circuits on such a LSI.
Moreover, the above described test of the Schmitt input circuit is performed with a LSI tester and the like, Therefore, another problem is that measurement accuracy of the threshold voltage is influenced by power power supply noise in the test environment (e.g. a test board and a test socket). Particularly, in a micro controller or the like having a original operating frequency about 10-30 MHz, GND and V.sub.DD ambient noise of about 0.1-0.3 V may occur. Thus, though the V.sub.TH and the V.sub.TL of the Schmitt input circuit are measured under such an environment, accurate values can not be obtained. Usually, a value larger than the true value is measured as the V.sub.TH, and a value smaller than the true value is measured as the V.sub.TL. To reduce these measuring errors, the noise supply is decreased by lowering the operating frequency while threshold voltages are measured. However functional tests take a longer time when the operating frequency is lowered.
As above described, the conventional integrated circuit requires a long time to test the Schmitt input circuit and the accuracy of the test result is poor. These problems result in cost increase.